1. Field of the Invention
The present invention generally relates to an apparatus for reproducing data and an apparatus for recording/reproducing data such as a magnetic disk apparatus, an optical disk apparatus, or a like, and more particularly to the apparatus for recording/reproducing data, which writes or reads data to or from a recording medium by using a turbo coding approach and an iterative decoding approach, and a device for reproducing data, which reads data from the recording medium by using the iterative decoding approach.
2. Description of the Related Art
A turbo coding approach is a coding technology having a greater coding gain and has been a subject of attention in the communication field. In general, a turbo coding device encodes a data bit sequence u by using two recursive convolutional encoders. For example, the turbo coding device can be configured as shown in FIG. 1 and FIG. 2.
In FIG. 1, a turbo coding device 10 includes a first encoder 11, an interleaver (π1) 12, a second encoder 13, and a multiplexer (MUX/Puncture) 14.
The first encoder 11 and the second encoder 13 are recursive convolutional encoders. The first encoder 11 generates a parity bit sequence p1 with respect to the data sequence u input to the turbo coding device 10. The interleaver (π1) 12 outputs a signal sequence in which a bit arrangement order of the data bit sequence u is changed. The second encoder 13 generates a parity bit sequence p2 with respect to a signal sequence from the interleaver (π1) 12.
The MUX/Puncture 14 generates an encoded data bit sequence yk by multiplexing the data bit sequence u with the parity bit sequence p1 output from the first encoder 11 and the parity bit sequence p2 output from the second encoder 13 in accordance with a predetermined rule. When multiplexing the data bit sequence u with the parity bit sequence p1 and the parity bit sequence p2, the MUX/Puncture 14 thins off (punctures) bits in accordance with a predetermined rule (a puncture function) so as to improve a coding rate. The encoded data bit sequence yk generated as described above is output from the turbo coding device 10. In a communication system, the encoded data bit sequence yk is modulated in accordance with a predetermined rule and is output from a transmitting device.
Alternatively, in the turbo coding device 10 configured as shown in FIG. 2 as a turbo coding device 11, two recursive convolutional encoders (the first encoder 11 and the second encoder 13) are connected in series. In this example, the data bit sequence u is encoded by the first encoder 11 and the bit arrangement order of a signal sequence obtained by encoding the data bit sequence u is changed by the interleaver (π1) 12. A signal sequence output from the interleaver (π1) 12 is encoded by the second encoder 13. Then, a signal sequence obtained by the second encoder 13 is output as the encoded data bit sequence yk.
When a signal transmitted from the transmitting device as described above is received as a receive signal by a receiving device, the receive signal is demodulated in the receiving device. Then, signal value sequences U, Y1, and Y2 are obtained with respect to a data bit sequence u and parity bit sequences p1 and p2 included in the encoded data bit sequence yk. The signal value sequences U, Y1, and Y2 are input to a decoding device corresponding to the turbo coding device 11.
In the decoding device, a soft output decode is conducted by two decoders corresponding to the two of the first encoder 11 and the second encoder 13. Soft output information (likelihood information) regarding each information bit obtained from one decoder is provided to another decoder as anterior information. And such an operation is repeatedly conducted. For example, such the decoding device is configured as shown in FIG. 3 so as to process the signal value sequences U as demodulated signal sequences corresponding the data bit sequence u and the parity bit sequences p1 and p2, respectively, included in the coded data bit sequence yk output from the turbo coding device 10 shown in FIG. 1.
In FIG. 3, a decoding device 20 includes a first SISO (Soft In Soft Out) decoder 21, interleavers (π1) 22 and 23, a deinterleaver (π1−1) 25, a second SISO decoder 24, and a hard decision decoder 26. The first SISO decoder 21 corresponds to the first encoder 11 and the second SISO decoder 24 corresponds to the second encoder 13.
When the first SISO decoder 21 receives signal value sequences U and Y1, the first SISO decoder 21 inputs the signal value sequences U and Y1 and also inputs anterior information L(u). Then, the first SISO decoder 21 conducts a MAP (Maximum a Posterior Probability) decoding. A posterior probability is a probability whether or not a bit uk is “0” or “1” in a condition in that a signal value sequence Y (y0, y1, . . . , yk, . . . , yn) is detected. In the MAP decoding, an LLR (Log-Likelihood Ratio) L(u*), which is a logarithm ratio of a posterior probability P (uk|Y), is calculated as follows:L(u*)=L(uk|Y)=ln{P(uk=1|Y)/P(uk=0|Y)}  (1).In the above calculation in accordance with an equation (1), the signal value sequence Y is the signal value sequences U and Y1.
A probability P(uk=1|Y) where the bit uk is “1” and a probability P(uk=0|Y) where the bit uk is “0” are calculated based on a trellis diagram showing a state transition obtained from the signal value sequences U and Y1.
On the other hand, the LLR L(u*) is calculated in accordance with the following equation (2):L(u*)=Lc*yk+L(uk)+Le(uk)  (2).
Lc*yk: channel value                (Lc: a constant (channel value constant) determined by S/N (Signal to Noise ratio), which is the ratio between the magnitude of a signal (meaningful information) and the magnitude of background noise,        yk: received signal series y0, y1, . . . , yn)        L(uk): anterior information as a known appearance probability related to uk=1, uk=0)        Le(uk): external likelihood information obtained by associating with uk from a constraint of a code.        
Based on the above equation (2), the first SISO decoder 21 calculates the external likelihood information Le(uk) in accordance with a following equation:Le(uk)=L(u*)−Lc*yk−L(uk)  (3).
The LLR L(u*) calculated as described in the above equation (3) is substituted (refer to the equation (1)) so as to obtain the external likelihood information Le(uk). A sequence of the external likelihood information Le(uk) sequentially obtained is supplied to the second SISO decoder 24 as a sequence of the anterior information L(uk) via the interleaver (π1) 23. In addition to the sequence of the anterior information L(uk), the signal value sequence U is input to the decoding device 20 is supplied to the second SISO decoder 24 through the interleaver (π1) 22, and a signal value sequence Y2 is input directly to the SISO decoder 24.
The second SISO decoder 24 calculates a new LLR L(u*) by considering the anterior information L(uk) input thereto according to the equation (1). And, the second SISO decoder 24 calculates the external likelihood information Le(uk) in accordance with the above equation (3) by using the LLR L(u*) and the anterior information L(uk) supplied from the first SISO decoder 21.
The external likelihood information Le(uk) obtained by the second SISO decoder 24 is supplied to the first SISO decoder 21 as the anterior information L(uk) via the deinterleaver (π1−1) 25. And, the first SISO decoder 21 calculates the LLR L(u*) and the external likelihood information Le(uk) in accordance with a procedure described above by considering the anterior information L(uk). Then, the external likelihood L(uk) from the first SISO decoder 21 is supplied to the second SISO decoder 24 as the anterior information L(uk).
As described above, the first SISO decoder 21 and the second SISO decoder 24 use the external likelihood Le(uk) calculated on another decoder as the anterior information L(uk) so as to repeatedly calculate the LLR L(u*) (iterative decoding). It should be noted that the anterior information L(uk) is “0” (L(uk)=0) at a first calculation of the first SISO decoder 21.
The hard decision decoder 26 decides whether or not the bit uk is “0” or “1”, based on the LLR L(u*) obtained by the second SISO decoder 24 when a decoding process described above is repeated a predetermined number of times. For example, the hard decision decoder 26 decides that the bit uk is “1” (uk=1) when the LLR L(u*) is positive (L(u*)>0). On the other hand, the hard decision decoder 26 decides that the bit uk is “0” (uk=0) when the LLR L(u*) is positive (L(u*)=0). A decision result of the hard decision decoder 26 is output as a decoding result uk.
While the decoding process is repeated (iterative decoding), a probability of a value (“0” or “1”) to be originally obtained is becoming higher and a probability of an opposite value is becoming lower (a difference between a probability of the bit uk to be “0” and a probability of the bit uk to be “1”). Accordingly, reliability of a decision by the hard decision decoder 26 is increased.
A turbo coding/decoding approach as described above has been applied to a data recording/reproducing apparatus such as a magnetic disk apparatus, an optical disk apparatus, or the like. For example, an example applying the turbo coding/decoding approach to the magnetic disk apparatus is proposed in “Performance of High Rate Turbo Codes on a PR4-Equalized Magnetic Recording Channel”, Proc. IEEE mt. Conf. on Communications, pp 947–951, 1998, edited by W. E. Ryan.
In the data recording/reproducing apparatus, the turbo coding approach is used for a recording system (write system) for writing data on a recording medium and the iterative decoding approach as described above is used for a reproducing system (read system) for reproducing data from the recording medium. By applying approaches such as these, it is possible to reproduce data recorded on the recording medium (the magnetic disk, the optical disk (including magneto-optical disk), or the like) at a high density with few data errors.
In the data recording/reproducing apparatus such as the magnetic disk apparatus, the optical disk apparatus, or the like, a commutative recording medium is used. Thus, a data decoding condition (a channel value constant Lc, a repetitive number, or a like) to which the data recording/reproducing apparatus applies for one recording medium is not always an optimum data decoding condition in another data recording/reproducing apparatus which reproduces data from the same recording medium. Also, the data decoding condition suitably applied to one recording medium is not always the optimum data decoding condition for another recording medium.
Moreover, when a different recording method is applied to a different region of one recording medium to record data (when a different detecting method detects data in the different region), the optimum data decoding condition is not always the same data decoding condition for data recorded on each region.
Furthermore, in a case in which data is written in a data decoding condition for writing data to a regular recording medium, to a recording medium whose characteristics are deteriorated, it is impossible to precisely reproduce data even if the iterative decoding is conducted based on a reproduction signal from the recording medium.